Embodiments of the present invention relate to video test and measurement equipment, and more particularly to picture quality measurements for video.
Video compression methods, such as MPEG-2 and H.264 process video a small portion of the picture at a time. These small portions of the picture are often identical sized rectangles called blocks. These blocks are pieced together in a block grid. In the case of lossy, block-based compression methods, blocking impairments manifest themselves within an individual processed block. These blocking impairments correspond to errors caused by loss in the compression method. The highest loss results in total loss of detail, the AC portion of the video signal, within the block. This leaves only a constant, or DC value, for each picture element within the block. Lowest loss corresponds to 100% of the AC portion, which corresponds to the detail, of the video remaining unchanged for each of the channels of video, such as RGB, YUV for example.
While H.264 and other video codecs have optional deblocking filters designed to smooth abrupt edges, which may result from different block DC values, at block boundaries, the blurring due to loss of AC within in a block is still a problem.
The video industry needs a repeatable, verifiable method for both objective measurement and prediction of the subjective rating of the video quality due to this loss. Prior methods have not had defined units of measure, and generally have attempted to measure objective and subjective impairment simultaneously, thus measuring neither. The lack of a traceably defined unit of measure has prevented the prior methods from providing measurement and verification of the accuracy of the measurement results.
The prior art tries to estimate discontinuities at the block boundaries and estimate visibility of these discontinuities, without traceable and verifiable units.
An example of a blockiness measurement that comes close to having traceable and verifiable units is described in Blind measurement of blocking artifacts in images, by Zhou Wang et al., Proc. IEEE Int. Conf. Image. Proc., Vol 3, pp 981-984, September 2000. As with many other methods in the prior art, this method uses a relatively computationally expensive, and complex, spectral analysis to uncover periodicity of the block edge energy along vertical and horizontal dimensions. Blockiness is measured as the power of an estimated ideal blockiness signal superimposed on the original signal. While the power can be normalized to give a fully defined unit, as opposed to one relying on the LSB as described, and a synthesized ideal blockiness signal could be generated and super-imposed on a test video signal to verify accuracy of detection, the definition of ideal blockiness signal is itself problematic, as actual blockiness impairments do not generally correspond to the ideal and often video signals have the same spectral signature that the power spectrum method is designed to detect as blockiness. Thus, even if this method were extended to be full reference, the artifact detected is an ideal artifact that has been defined in a somewhat arbitrary way that relies primarily on block boundary discontinuities, which are commonly mitigated via de-blocking filter such as those included in the H.264 standard and sometimes incorporated in other encoder and decoder designs.
Also, for prediction of visibility and corresponding subjective quality rating, this method is illustrative of the prior art in general. A mask estimation is done as a very rough estimation of human vision response, again without traceable and verifiable units, or accuracy. In effect, visibility is not estimated directly, but rather relative visibility is estimated.
Other prior art methods exist with variations on the theme of finding block edges in the time, or frequency, domain, estimating the edge amplitude and taking the norm of these amplitudes, with or without masking estimates.
Automated methods of measuring DC blockiness with full reference and no reference would be useful. These methods could include compression information such as exact block boundary locations, or not. These methods would be desirable if they had traceable and verifiable units and accuracy, with results being either fully objective, or fully predictive of subjections assessment of the DC blocking impairments. It would also be desirable that the method have computational efficiency, such that it would have relatively low processing overhead for a given level of accuracy.